The invention relates generally to methods and systems for passing signals between a surface unit and downhole tools disposed in a borehole penetrating a subterranean formation.
The lower portion of a drill string for drilling a borehole in a subterranean formation is typically referred to as a bottomhole assembly. In general, the bottomhole assembly includes downhole tools that perform various downhole operations in the borehole. It is often necessary to send commands to one or more of these downhole tools in order to control operation of the downhole tool. For example, the bottomhole assembly may include a rotary steerable system that allows a borehole to be drilled in a formation directionally. To set the direction and inclination of the borehole segment being drilled, a command is sent from a surface location to the rotary steerable system.
In another example, the bottomhole assembly may include various formation evaluation tools, such as a logging-while-drilling (LWD) tool or measurement while drilling (MWD) tool designed to measure formation parameters. Certain formation evaluation tools, such as a formation pressure while drilling tool as described in US Patent Application No. 20050109538, may also be used to measure pressure using a probe that extends to contact the formation. For this operation, the pressure in the probe is momentarily brought below the formation pressure to draw formation fluid into the probe. Once the probe stabilizes at the formation pressure, the probe is retracted. These formation evaluation tools typically require commands to be sent from a surface location to the downhole tool. Typically, commands are sent to downhole tools using a telemetry system, such as a mud pulse system that manipulates flow of drilling mud through the drill string to create pressure pulses. This generally requires that the rate of surface mud pumps is adjusted manually, a process that can take several minutes and interferes with the drilling process.
MWD tools are typically provided with a telemetry component adapted to communicate with a surface unit. The telemetry component may be a mud pulse, electromagnetic (EMAG), acoustic or other telemetry device. In cases involving MWD tools having EMAG telemetry, the MWD-EMAG telemetry tools use relatively low frequency EMAG waves to communicate from a downhole location to a surface location. A typical MWD-EMAG telemetry tool includes a drill collar having an insulated gap and circuitry that creates a modulated voltage across the insulated gap. See, for example, U.S. Pat. No. 4,348,672. If the MWD-EMAG telemetry tool is included in a bottomhole assembly, the voltage across the insulated gap typically results in a large electric current flow along the drill string near the MWD-EMAG telemetry tool. Some current typically also flows through the earth and produces a weak electric field that is detected at the surface with two or more electrodes driven into the ground.
MWD-EMAG telemetry tools can be configured to receive signals from the surface via electric currents generated at the surface. These received signals may be communicated to other downhole tools in the bottomhole assembly if the MWD-EMAG telemetry tools can communicate with these downhole tools. One possibility is for internal or external wire links to be formed between an MWD-EMAG telemetry tool and other downhole tools to enable transmission of signals. However, it is sometimes impossible or impractical to run wires between downhole tools in a bottomhole assembly. For example, in a bottomhole assembly including a rotary steerable system, a mud motor may be positioned between the MWD-EMAG telemetry tool and the rotary steerable system. Passing a wire through the mud motor and connecting the wire to tools below the mud motor would be very difficult since the mud motor shaft rotates at a high speed and is attached to collars and/or the drill bit. A rotating connector would be required to make the wire connection, but such a rotating connector is unlikely to be reliable. Other methods of communicating through a mud motor can be complex (see, for example, U.S. Pat. No. 5,160,925) and may be unavailable on standard commercially available motors.
There are other examples where it may be cumbersome or impossible to form internal or external wire links between the MWD-EMAG telemetry tool and other downhole tools in a bottomhole assembly. For example, a typical power-drive rotary steerable system has a control unit that is held geostationary while the drill collar containing the control unit rotates about the control unit. In this case, running an electrical connection from the MWD-EMAG telemetry tool to the drill collar and control unit would be very difficult. The connection between the rotating drill collar and the geostationary control unit would require a rotating connection, which is unlikely to be reliable in a borehole environment. In cases where purely mechanical hardware, such as under-reamers and jars, are placed between the MWD-EMAG telemetry tool and a downhole tool, these mechanical hardware would likely have to be wired as well.
In another example, the bottomhole assembly may include a LWD seismic tool having an array of geophones or hydrophones for detecting seismic waves. These seismic sensors (geophones or hydrophones) are typically required to be placed 60 to 70 feet apart along the drill string and can acquire data only when the drill string is stationary and when the mud pumps are off, as described in, for example, U.S. Pat. No. 6,308,137. An MWD-EMAG telemetry tool could be useful in this case if it can communicate with the LWD tool. For example, the MWD-EMAG telemetry tool could detect the desirable conditions for LWD seismic measurement, i.e., stationary drill string and no mud circulation, and could communicate this to the seismic sensors in the LWD tool so that the seismic sensors can make the measurement. However, it would be impractical to run the long wires needed to make the signal transmission links between the MWD-EMAG telemetry tool and each of the seismic sensors in the LWD tool.
From the foregoing, it would be desirable in many situations to have a wireless telemetry system to transmit signals between an MWD-EMAG telemetry tool and other downhole tools in a bottomhole assembly or as a backup for other communications systems, such as wired systems. Wireless telemetry systems have been used in a bottomhole assembly. In one example, electromagnetic induction is generated using coils wrapped around drill collars, as described in U.S. Pat. No. 6,057,784. In another example, transformer coupling are formed using toroids mounted externally on drill collars, as described in U.S. Pat. Nos. 5,359,324 and 5,467,832. These wireless telemetry systems work well, but adding either type to an MWD-EMAG telemetry tool and other downhole tools, such as a rotary steerable system, in a bottomhole assembly would significantly increase the cost of the bottomhole assembly, increase the length of the bottomhole assembly, and add components to the bottomhole assembly that can easily fail. A wireless telemetry system that enables communication between the EMAG telemetry tool and downhole tools without these drawbacks may be beneficial.